The present Applicant has previously described printhead assemblies, which include a printhead (usually comprised of one or more printhead integrated circuits) and an ink supply manifold for supplying ink to the printhead. The printhead may be bonded to the ink supply manifold with an adhesive film. The printhead, the ink supply manifold and the adhesive film define ink pathways for supplying ink to nozzle openings defined in an ink ejection face of the printhead.
It is generally desirable for ink pathways to have hydrophilic surfaces. Hydrophilic surfaces improve printhead priming as well as print quality. During the operation of conventional printhead assemblies, there has been observed a phenomenon where bubbles form on the surfaces of the ink paths as ink flows therethrough. The formation of such bubbles causes blockages in the ink flow, reduces the wettability of the surfaces, and degrades print quality.
To ameliorate this problem, the surfaces of a printhead assembly may be activated with a plasma species during or after fabrication. Plasma activation of the internal surfaces of the printhead assembly renders these surfaces more hydrophilic and increases their wettability; this in turn inhibits bubble formation.
The hydrophilic characteristics conveyed to surfaces by plasma activation, however, degrade or relax with time. In the case of printhead assemblies, one solution for ameliorating this problem is to prime the printhead assemblies with ink, or an ink like fluid, after the surfaces of ink paths have been plasma activated, and to ensure that the printhead assemblies remain primed with ink (or the ink like fluid) until they are used. Keeping a printhead assembly primed with ink, or an ink like fluid, from the time of production until the time of use, however, introduces significant complexities, including the storage and transport of such primed printhead assemblies.
Sheu et al (Polymer Surface and Interfaces: Characterization, Modification and Application, 1997, pp 83-90) describe treatment of plasma activated surfaces with a polyethyleneimine (PEI) solution in order to retard relaxation of the plasma activated surface. According to the current understanding in the art, PEI relies predominantly on interactions with carboxylate groups on the activated surfaces. PEI is therefore understood to be less effective when used on surfaces activated with a plasma other than a carbon dioxide plasma.
U.S. Pat. No. 5,700,559, U.S. Pat. No. 5,807,636, and U.S. Pat. No. 5,837,377 describe a hydrophilic article for use in aqueous environments, including a substrate, an ionic polymeric layer on the substrate, and a disordered polyelectrolyte coating ionically bonded to the polymeric layer.
The plasma activation of a printhead assembly is conventionally performed using a vacuum plasma processing method. Vacuum plasma processing methods, however, are expensive and time consuming. A vacuum plasma processing method requires costly and specialised equipment to create a vacuum and to generate a plasma within the vacuum. Further, significant time is required for loading and unloading a work piece into/from a vacuum chamber, creating and releasing the vacuum, and allowing the plasma to diffused through and activate the work piece.
A further disadvantage associated with vacuum plasma processing is that vacuum plasma processing is indiscriminate insofar as which surfaces of the work piece are activated, and to what extent they are activated. Directed activation of specific surfaces is generally difficult to achieve and the selective activation of internal surfaces alone is impossible.
Still further, the vacuum plasma processing method does not complement serial/assembly-line type production process commonly used in the fabrication of printhead assemblies. To enable the vacuum plasma process to be cost feasible, printhead assemblies are processed in batches. The collation and later de-collation of printhead assemblies into batches for vacuum plasma processing interrupts the work flow of serial, assembly-line type production processes and reduces the efficiency of the production process.
Quality control issues also arise from the discontinuity caused by the batch processing of printhead assemblies for vacuum plasma processing. A first printhead assembly removed from a vacuum processing batch and a last printhead assembly removed from the same batch vary in age. For example, a printhead assembly removed first from the batch exiting the vacuum plasma process has an active surface that is “younger” than a printhead assembly removed last from the same batch. Such differences in age affect the results of further processing steps performed downstream of the vacuum plasma processing step.
Accordingly, it would be desirable to provide a method for hydrophilizing surfaces of ink paths in printheads and/or printhead assemblies.